1. Field of the Invention
The present invention generally relates to implantable medical devices, and more particularly, relates to implantable medical devices for the administration of a drug.
2. Description of the Prior Art
It has been known for some time to implant drug administration devices. Such devices employ a reservoir which is filled by the attending medical personnel. The drug material, usually a liquid, is dispensed within the body of a patient over time. This dispensing may be accomplished at a constant rate associated with the pressure under which the liquid drug is held within the reservoir. Alternatively, the dispensing may be at a programmed rate using a microprocessor driven pump.
A common problem associated with the various designs of implantable drug administration devices is that associated with over filling. In normal operation, the reservoir is filled percutaneously from a hypodermic syringe. The medical person filling the reservoir has historically needed to rely upon tactile pressure to determine the state of fill of the reservoir. This problem is made all the more difficult as the various designs have used fluids to back-fill the volume of the reservoir in an attempt to maintain a constant pressure within the reservoir, independent of remaining drug volume.
The result is that the filling process occurs at an almost constant pressure until the reservoir is completely filled. Any additional liquid injected may cause severe damage to the drug administration device, and leakage to the medical detriment of the patient. The difficulty with prior art drug administration devices is the lack of a precise and reliable over fill protection device to prevent damage to the drug administration device and uncontrolled leakage.